I don't know how to crochet/knit, but did you try looking for bear or Build-a-Bear-sized patterns?
For the most part, 18" doll patterns would make clothes too small for the 17" Duffy and Shellie May, as the bears are much thicker than the dolls (18" doll-sized shorts or short-sleeved tops might fit the 12" bears).
I recently made a sweater for my Duffy using this pattern on Ravelry. I plan to make a few more to either sell or give away, because it was a very fun pattern and knit up pretty fast!
Typically, you can resize patterns as long as you know what your gauge of stitches per inch is. I made the pattern as written and while it did come out fitting loosely, it is meant to be like that. It knit up very quickly, and I'm sure you can find other teddy bear clothes by searching on Ravelry or other similar websites. I doubt you will find much more than sweaters, though.
Here's a picture of him in the finished sweater (pardon the image quality):
Welcome, setavulos! That sweater looks wonderful. Nicely done! And thanks for the link — I almost didn't notice that "this pattern" is clickable hypertext. Ravelry looks fun. I've only knitted once in my life, though, and it was just a scarf.
I've been looking for other free patterns, but I haven't had much luck finding some other patterns to share that aren't just sweaters. I did, however, decide that if anyone was interested in clothing, that I might try to design some items and provide the patterns here and on Ravelry. That would have the added bonus of letting me make clothes for my own Duffy, so I don't think that is too bad a deal! So if there is an item that someone would like to see I can take a crack at it!
Additionally, I have a few more patterns for you all: March Teddy, which is only available for free download for a few more days and is a pattern for an 8" bear Monkster Gets Stranded, which is made for a 16-18" bear